A cable network delivers services such as digital television, Internet, and Voice-over-IP (VoIP) phone connection. The services are delivered over a tree-like network of a broadband coaxial cable termed a “cable plant”. Digital television signals are broadcast from a headend connected to the trunk of the cable plant, and delivered to subscribers' homes connected to the branches of the cable plant. In going from the headend to the subscribers, the signals are split many times, and are attenuated in the process. A strong downstream broadcast signal is required to ensure a strong enough signal level at the subscribers' premises.
Internet and VoIP services use signals directed from the subscribers' premises back to the headend, or “upstream” relative to the broadcast signal. The tree-like structure of the cable plant ensures that the upstream signals are brought together into the common trunk connected to the headend. Time-division multiple access (TDMA) is used for each upstream frequency channel to ensure that the upstream signals at a same channel frequency do not interfere with each other as they are combined.
Unfortunately, not only the upstream signals, but also noise can propagate in the upstream direction. The noise originates at customers' premises due to improper cable grounding or shielding, non-professional equipment installation, loose connectors, unshielded indoor equipment such as electrical motors, TV sets, and the like. Old or improperly configured cable modems can also contribute to upstream noise, by emitting at frequencies outside of assigned channel frequency, e.g. harmonics of a main emission frequency band can be generated due to nonlinearities of the modem's output amplifier, oxidized cable connectors and splitters, etc. This ingress noise is particularly problematic in the upstream direction, because as it propagates from many end locations towards the common trunk of the cable plant, it tends to accumulate and grow in magnitude, compromising or even completely disabling digital communications, at least for some subscribers.
A further problem for the upstream direction is that the upstream signals occupy a lower frequency band, typically from 5 MHz to 45 MHz, as compared to the downstream signals spanning typically from 50 MHz to 1 GHz. Thus, the upstream signals are closer in frequency to ingress noise, which tends to be a low-frequency noise. One typical source of upstream noise is so called “common path distortion” or CPD, which appears at beat frequencies of a powerful downstream signal, which are generated on nonlinear elements such as oxidized connectors. The signal at beat frequencies propagates in the upstream direction, contributing to the ingress noise. Other types of ingress noise include interference from power lines, electrical motors, radar equipment, etc. Different types of ingress noise have different spectral characteristics.
An insight into possible sources of ingress noise can be gleaned by measuring spectral behavior of the upstream signal. Once a type of ingress noise is identified, a technician may be dispatched to locate and eliminate the source of the ingress noise. The technician usually travels along the cable plant, making ingress noise measurements on each leg of a bridge amplifier, and proceeding to a next location corresponding to the “noisiest” leg of the amplifier.
Because the ingress noise troubleshooting can take many hours of technician's work, which sometimes extends for days, various methods have been suggested to alleviate the noise problem for as many customers as possible, at least for the time while the source of the ingress noise is located and dealt with. By way of example, Hsu et al. in US Patent Application Publication 2004/0203392 and Howard in US Patent Application Publication 2006/0141971 disclose a method for maintaining an upstream communication in presence of CPD ingress noise. The method includes detecting the CPD ingress noise and constraining upstream transmission parameters to exclude the CPD frequencies. The CPD ingress noise is detected in Hsu and Howard systems by performing a fast Fourier transform (FFT) of the upstream signal and looking for CPD spectral patterns. Detrimentally, the constrained upstream transmission parameters may reduce the available upstream transmission bandwidth, so that a subsequent identification and elimination of the CPD sources is still required. The previously measured CPD spectra are of a limited value for this purpose, because the noise spectra are time-varying, and varying from location to location of a cable plant; this spatial and temporal ingress noise variability represents a serious challenge for cable network service providers. Furthermore, CPD has become more difficult to tell apart from other types of ingress noise, because the downstream channels moved from analog to digital encoding, and as a result no longer have a constant frequency.
Naegeli et al. in U.S. Pat. No. 6,895,043 disclose a method and an apparatus for measuring “quality” of upstream signals. A cable network headend assigns a normal time slot to a cable modem being tested. An FFT engine obtains an upstream signal spectrum during this time slot. A dummy time slot, not assigned to any cable modem, is created, and the FFT engine obtains an upstream signal spectrum during the dummy time slot as well. The two spectra are then compared to each other. Through this comparison, undesirable noise spurs, caused by the cable modem being tested, can be detected. For example, out-of-band frequency harmonics of an aged output amplifier and/or connectors of the cable modem can be detected.
Detrimentally, in the method of Naegeli, one can only get an update during the ranging time slot. If there is only a ranging time slot once per second, then the update will be once per second, and a chance of catching noise will be small. Another drawback results from having to request a dummy time slot as a reference. The cable modem termination system (CMTS) is often configured to keep statistics related to modem quality. A dummy slot may be shown as a lost transmission in these statistics. As a result, the node being tested may be inadvertently flagged by the CMTS as a “poor” node. Furthermore, a modem is needed to be able to request a packet. This increases the electrical power requirement for a field instrument, degrading battery life and usage time.
It is noted that the prior-art methods of upstream signal spectral measurements share a common drawback of not being tied to a particular upstream transmission packet emitted by a modem under test. The ingress noise is not constant in time, often being sporadic and/or pulsed in nature. Accordingly, the measured upstream spectra may not be representative of problems with an upstream data transmission by a particular cable modem.